1. Technical Field
The present invention relates to liquid crystal displays (LCDs), and more particularly to an LCD with a periodical changed voltage difference between a data voltage and a common voltage. The present invention also relates to a driving method of the LCD.
2. Description of Related Art
A liquid crystal display (LCD) utilizes liquid crystal molecules to control light transmissivity of each of pixels of the LCD. The liquid crystal molecules are driven according to external video signals received by the LCD. A conventional LCD generally employs an inversion driving method to drive the liquid crystal molecules to protect the liquid crystal molecules from decay or damage.
FIG. 11 is a side view of a conventional LCD. The LCD 10 includes a first substrate 11, a common electrode 12, a first alignment film 13, a liquid crystal layer 14, a second alignment film 15, a plurality of pixel electrodes 16, and a second substrate 17. The first substrate 11 is opposite to the second substrate 17. The common electrode 12 is disposed on an inner surface of the first substrate 11. The plurality of pixel electrodes 16 are disposed on an inner surface of the second substrate 17 and arranged in a matrix. The first alignment film 13 is coated on the common electrode 12, and the second alignment film 15 is coated on the plurality of pixel electrodes 16. The liquid crystal layer 14 is sandwiched between the first alignment film 13 and the second alignment film 15. Each of the pixel electrodes 16, part of the common electrode 12 opposite to the corresponding pixel electrode 16, and liquid crystal molecules (not labeled) sandwiched therebetween cooperatively define a pixel unit (not labeled).
Data voltages generated by a data driving circuit (not shown) are provided to the plurality of pixel electrodes 16, and a common voltage generated by a common voltage generating circuit (not shown) is provided to the common electrode 12. In each pixel unit, an electric field is generated between the pixel electrode 16 and the common electrode 12. The electric field controls rotating angles of the liquid crystal molecules of the pixel unit, whereby the rotating angles determine the light transmissivity of the pixel unit. The light transmissivity of the pixel unit determines a brightness of the pixel unit. The LCD 10 displays images via controlling the brightness of each of the pixel units.
A waveform diagram of the data voltage and the common voltage of one of the pixel units is shown in FIG. 12. In frame N−1, a value of the data voltage is Vdata1, a value of the common voltage is Vcom, where Vdata1>0, Vcom>0, Vdata1<Vcom. A value of the electric field of the pixel unit is (Vcom−Vdata1)/d, where d is a vertical distance between the common electrode 12 and the pixel electrode 16. A direction of the electric field of the pixel unit is from the common electrode 12 to the pixel electrode 16. In frame N, the value of the data voltage is Vdata2, the value of the common voltage is Vcom, where Vdata2>Vcom, Vdata2−Vcom=Vcom−Vdata1. The value of the electric field of the pixel unit is (Vdata2−Vcom)/d. The direction of the electric field of the pixel unit is from the pixel electrode 16 to the common electrode 12. In frame N+1, the value of the data voltage is Vdata1, and the value of the common voltage is Vcom. The value of the electric field of the pixel unit is (Vcom−Vdata1)/d. The direction of the electric field of the pixel unit is from the common electrode 12 to the pixel electrode 16. The value and the direction of the electric field of the pixel unit in frame N+1 are the same as that in frame N−1. That is, frame N−1 and frame N define a minimum period. The value and the direction of the electric field of the pixel unit in the following frames repeat that in frame N−1 or frame N.
The direction of the electric field of each pixel unit is alternate in each two continuous frames, but the value of the electric field of each pixel unit is constant in each frame. The rotating angles of the liquid crystal molecules of each pixel unit are merely determined by the value of the electric field of each pixel unit. That is, when the value of the electric field of the pixel unit is constant, the rotating angles of the liquid crystal molecules of the pixel unit are constant.
In fact, the liquid crystal layer 14 is not pure and has a plurality of impurity ions (not shown). The alignment films 13 and 15 are made of organic materials and easily capture the impurity ions. When the value of the electric field of each pixel unit keeps constant for a long time, the rotating angles of the liquid crystal molecules of each pixel unit are constant, correspondingly. That is, each liquid crystal molecule stays in the same position in the liquid crystal layer 14. A moving resistance stressed by the liquid crystal molecules to the impurity ions has little effect on random motions of the impurity ions. Thus, part of the impurity ions are captured by the alignment films 13 and 15 and a residual direct current electric field (not shown) is generated between the first alignment film 13 and the second alignment film 15. Even if the value of the electric field of each pixel unit changes, the residual direct current electric field may still exist. The residual direct current electric field also controls the liquid crystal molecules to rotate, and an extra rotating angle of each liquid crystal molecule exists. If the value of the electric field of each pixel unit changes in a small range, the liquid crystal molecules may stay in the same position as in previous frames. Thus, images of the previous frames still can be watched, which is so-called image residue phenomenon.
It is desired to provide an LCD which overcomes the above-described deficiencies. It is also desired to provide a related driving method for an LCD.